PARTICLE NUCLEATION DURING MICROEMULSION POLYMERIZATION OF METHYL-METHACRYLATE

The evolution of the reaction conversion and particle size distributio n during the microemulsion polymerization of methyl methacrylate (MMA) is used to determine the particle nucleation mechanisms. A pseudo-3-c omponent oil-in-water microemulsion is formed with water, MMA, and a m ixture of dodecyltrimethylammonium bromide (DTAB) and diodecyldimethyl ammonium bromide (DDAB) in a 3:1 weight ratio as surfactant. Polymeriz ation is initiated with either an oil-soluble or a water-soluble initi ator and conversion followed either by measurement of the unpolymerize d monomer concentration in samples taken during the reaction or by dir ect on-line densimetry. A two-stage process is observed. The first sta ge, described by a very slow increase in conversion, is attributed mai nly to homogeneous nucleation, and the second stage, characterized by a much higher rate of conversion, involves continuous nucleation and i s governed mainly by a micellar-entry mechanism.